Cal and Alk use differences...

[GOSKN5]

So I have a 120 gallon that is about 6 months old.. some of the corals are older from tank transfer (upgrades)

I started noticing alk consumption was picking up several months ago and I actually had an Alk drop and lost a few chalices...

So I started testing more and dosing consistently... I started off adding roughly 3ml Cal and 4ml alk.. this remained the same for about a month or so...

I added a few SPS that have started to encrust and grow... I skipped a week of testing and but still dosed... Tested last night and my alk had dropped significantly.. from around 10 to 8.6.. I dosed to raise it back.. Tested tonight and same drop... 10 to around 8.6..

My calcium didn't usage didn't increase.. still roughly dropping from 450 to 425 in a 25 hour time frame...

Does anything sound off? Mg tested at 1450...

So roughly adding 20ml of alk and only 5ml of calcium to keep my levels where I want em.. any thoughts??

 

[Downbeach]

Have you taken your WC schedule into consideration? The uptake of Ca and Alk, by “stony” type coral, coralline algae, etc. in a reef system is 20 ppm Ca to 50 ppm(2.8dKH) Alk(CaCO3), which is a ratio of 2 pts Ca to 5 pts Alk (2:5). Equal doses of any two part supplements are designed to provide the same ratio's. BUT, once you factor in a WC schedule, you may find that has skewed your dosing due to the fact that WC's are better able to keep up with a particular Ca and Mg demand.
There's a procedure I think it prudent to follow to help to minimize the effects of WC's on dosing. Although, I think it good practice at any time.
First, if you're not using the entire package, would be to thoroughly mix the salt mix to make sure there was no elemental stratification that may have occurred during shipping, and you're getting the proper proportions of all the elements, i.e. Ca, Mg, Alk, etc., with each measure of salt mix when you make up your WC water.
Secondly, following proper mixing procedures, i.e. using RO/DI water, adding the salt mix to the water, etc., I would make sure to accurately get the SG you want, I preferred 1.026(35ppt), using a properly calibrated measuring device, i.e. refractometer calibrated with a 35ppt solution(not plain water), to ensure the same amount of these elements with each WC. Using a swing arm type of hydrometer, or other non/mis calibrated device can cause differences in your SG when you make up your WC water, and further skew a dosing schedule. Using a decent set of test kits, i.e. Salifert, Red Sea Pro, etc., measure your Mg first, making sure it’s between 1200-1400ppm, then your Alk and Ca.
Next, I would set and keep a schedule of WC's in terms of both frequency, i.e. weekly, bi monthly, monthly, etc. and volume, i.e. 10%, 15%/10 gal.,15 gal., etc. to help minimize any skewing that may normally occur, since this is where you're adding/replenishing those elements in an ongoing basis.
Depending on a particular load in any system, it's likely that you may need to adjust your dosing based on a particular WC schedule, i.e. the disproportionate use of Alk vs Ca supplement, since the WC is better at keeping up with the Ca needs. You may notice that, with regular WC's, you may never have to dose any Mg, since there is very little of it used (about 1/10th of your Ca use), and it’s replenished with regular WC's.

Another good read:
http://reefkeeping.com/issues/2004-12/rhf/index.php

 

[GOSKN5]

So I do roughly 10 gallons a week for wc.. occasionally I do 5 gallons, and once every month or two I might skip a week and just syphon detrius (BB tank)...

I am mixing correctly etc, using refractometer..

 

[Nibejeebies]

following along with this.

 

[Randy Holmes-Farley]

A drop of 1.4 dKH in alkalinity will be accompanied by a drop in calcium of about 9 ppm calcium.

I doubt in a few days you can readily distinguish calcium demand to better than +/- 10 ppm per day, so I do not see any concerns of reason to think the demand is out of whack. :)